Sense 1onset, oncoming -- (the beginning or early stages; "the onset of pneumonia") => start -- (the beginning of anything; "it was off to a good start")

Sense 2attack, onslaught, onset, onrush -- ((military) an offensive against an enemy (using weapons); "the attack began at dawn") => operation, military operation -- (activity by a military or naval force (as a maneuver or campaign); "it was a joint operation of the navy and air force")